1. Field of the Invention
The present invention-relates to a photoconductor for electrophotography that has an intermediate layer between an electrically conductive substrate and a photosensitive layer to stably obtain excellent electric properties and excellent image qualities under the condition of repeating the image-forming process.
2. Description of the Prior Art
From the time that the Carlson Process was invented by C. F. Carlson in 1938, technologies associated with electrophotography have been rapidly progressed and used in the data-processing systems such as photocopying machines, laser printers, light-emitting diode printers, facsimile, and so on.
The Carlson process is known as the electrophotographic process for image formation, that comprises the steps of:
(i) providing charges uniformly on a surface of the photosensitive member by means of corona discharge in the absence of light; PA1 (ii) exposing a charged surface of the photosensitive member to light to form a latent image that is a charge pattern on the photosensitive member that mirrors the information such as characters and figures to be transformed into the real image; PA1 (iii) developing the latent image by applying toner particles that are brought into the vicinity of the latent image to obtain a toner image; and PA1 (iv) transferring and fixing the developed toner image on a support medium such as a sheet of paper and plastics, following that the photosensitive layer is discharged and cleaned of any excess toner particles using coronas, lamps, and brushes and scraper blades, or both. Consequently, the image formation can be repeated by using the same photosensitive member. PA1 an electrically conductive substrate; PA1 a photosensitive layer formed on the electrically conductive substrate; and PA1 an intermediate layer formed between the electrically conductive substrate and the photosensitive layer, wherein PA1 0.1-10 mol of an acid selected from the second organic acid, the inorganic acid, and a mixture thereof with respect to 1 mol of the organic acid selected from a group of aromatic sulfonic acid, alicyclic sulfonic acid, and mixtures thereof. PA1 a compound selected from a group of indole derivatives, enamine compounds, amine compounds, hydrazone compounds, styryl compounds, butadiene compounds, triphenylmethane compounds, and pirazoline compounds; and PA1 a binder resin which is mutually soluble to the compound, preferably selected from a group of polycarbonate resin, polystyrene resin, styrene resin, and acrylate resin.
The photosensitive member described above is generally called as an electrophotographic photoconductor and is responsible for the photosensitive function such as increase in electrical conductivity during the light exposure in the field of the electrophotography. The photoconductor is generally formed by laminating photoconductive insulating layer approximately 5-200 .mu.m in thickness on an electrically conductive substrate to form a photoconductor. In this case, the electrically conductive substrate acts as an electrode in the photoconductor. In the case of using the step of toner-image transfer in the process of image formation, it is important that the electric properties of the photosensitive layer should be kept at a constant all the times in spite of after repeating the steps of discharge, exposure, development, image-transfer, and cleaning.
Taking a mechanical strength of the photoconductor in consideration with the above maters, therefore, the conventional photoconductors have been mainly prepared by inorganic photoconductive materials such as selenium, selenium alloys, zinc oxide, and cadmium sulfate. In recent years, however, there have been much more studies for using organic photoconductive materials by virtue of their advantageous features for preparing a light-weighted flexible layer cost effectively without causing any toxic effects.
Furthermore, the photosensitive layer of the photoconductor has been classified into two types in general. That is, one is formed as a single layer (hereinafter, referred as a mono-type photosensitive layer) and the other is formed as a laminate of layers which are functionally distinguishable (hereinafter referred as a laminate-type photosensitive layer). The laminate-type photosensitive layer comprises a lower layer for the function of generating charge carriers and an upper layer for the function of transporting the charge carriers. These layers are easily prepared and modified by selectively using appropriate raw materials to improve their physical properties such as photosensitivity against specific wavelength of illuminated light and sensitivities against the spectrum according to the range of wavelengths of the illuminated light.
Accordingly, the laminate-type photosensitive layers have been much more studied that the mono-type one and used in many kinds of the electrophotographic devices such as photocopying machines, facsimile machines, and printers.
More recently, there have been much demands for miniaturizing the electrophotographic devices and also for increasing their printing speeds. For replying to these demands, the miniaturized devices should be manufactured so as to keep their abilities of providing good image qualities by forming the image at the same speed or at the higher speed compared with that of those currently in use. In this case, therefore, it is also necessary to miniaturize a drum of the miniaturized device on which the photoconductor will be mounted.
For performing the printing and coping at least at the same speed as that of those currently in use, the small-sized drum must rotate at a higher rate compared with the conventional one.
In the miniaturized device, accordingly, the photoconductor provided on the small-sized drum can be used more frequently than that of the conventional one, so that the photoconductor to be installed in the miniaturized device should be improved so as to have a high durability against repetitive usage and a good respond sensitivity to the illuminated light.
Several photoconductors have been proposed in order to reply these demands. Most of them include more than one layer in their photosensitive layer portions. That is, the photosensitive layer has functionally distinguishable layers: one contributes to generate an electric charge by absorbing illuminated light; and the other contributes to transport the electric charge. The charge generation layer mainly includes a charge-generating material, while the charge transport layer mainly includes a charge-transporting material. Besides the charge-transporting material, the charge transport layer optionally includes a binder or a stabilizer, or both. The binder is responsible for forming a membrane structure while the stabilizer is responsible for stabilizing the membrane structure by arresting the progress of oxidation to be caused by ultraviolet light, ozone, or the like.
Japanese Patent Application Publication No. 55-42380 discloses a photoconductor that has functionally distinguishable layers, a charge generation layer and a charge transport layer, and in this case the farmer includes Chlorodiane Blue as a charge-generating material and the latter includes a Hydrazone compound as a charge-transporting material. Both the response rate of the photoconductor and its durability to last tens of thousands of cycles of the image formation are mainly depended on the nature of the charge transport layer. Accordingly, several materials have been proposed as a raw material for preparing the charge transport material, for example Pyrazorine derivatives (the Journal of Photographic Science and Engineering vol. 21, No. 2, page 73, 1977); Enamine derivatives (the Journal of Imaging Science vol. 29., No. 1, page 7, 1985 and Japanese Patent Application Laying-Open No. 63-170651); and Benzidine derivatives (Japanese Patent Application Laying-Open No. 3-43744 and Japanese Patent Application Laying-Open No. 59-9049). In spite of these investigations, however, these charge-generating materials do not satisfy the request of providing the photoconductors with both an excellent durability to the repetitive usage and a good response speed to the illuminated light.
Consequently, a photoconductor practically used at the present time is generally in the type of organic photoconductor that has functionally distinguishable layers: a charge generation layer and a charge transport layer which are laminated on an electrically conductive substrate in that order. The photoconductor is prepared by the process including the steps of: performing sublimation or vapor deposition of the organic charge-generating material on the electrically conductive substrate, or applying and drying a coating solution prepared by dispersing and dissolving with a binder in an organic solvent on the electrically conductive substrate to form the charge generation layer; and applying and drying another coating solution prepared by dispersing and dissolving a charge-transporting material with a binder in an organic solvent on the charge generation layer to form the charge transport layer. The photoconductor that has the laminate structure thus obtained is enough to perform the process of image formation. In the practical use, however, it is important to form images without any defects and also it is important to keep good image qualities during the period of repeating the usage. Therefore the photoconductor should be formed as a uniform structure without any defects to obtain stable electric properties thereof and sufficient durability to last of tens of thousands of cycles of the image formation.
By the way, the function of the charge generation layer is to absorb the illuminated light and generate electron carriers. These electron carriers move quickly to both the electrically conductive substrate and the charge transport layer. It is required that the charge generation layer does not trap free carriers during their movements for injecting them into the electrically conductive substrate and the charge transport layer. Therefore, it is preferable that the charge generation layer is formed as thin as possible. The charge generation layer used in the conventional photoconductor is generally formed as a thin film with a thickness of in the order of sub microns, so that the charge generation layer is easily affected by troubles on a surface of the electrically conductive substrate, such as unstable electric properties, an irregular shape, impurities, and roughness thereof. However it is difficult to make the substrate without causing the troubles described above. Consequently these troubles affect the photosensitive layer to deteriorate the image qualities by causing whiteness of non-imaged areas and blackness and non uniform appearance of the imaged areas in the copy.
In general, the electrically conductive substrate is formed as a drawn cylindrical tube of aluminum alloy, or a cylindrical tube having a surface smoothed by means of cutting, grinding and polishing, but it is difficult to avoid the troubles described above.
Up to the present time, the conventional photoconductor has been modified by providing an intermediate layer between the photosensitive layer and the electrically conductive substrate for obtaining a smooth and uniform surface of the charge generation layer and also for suppressing the deterioration of charge-holding properties of the photoconductor. The deterioration can be caused by injecting holes (which are required for converting light to electron-hole pairs) from the electrically conductive substrate to the photosensitive layer. In this case, the intermediate layer is typically made of an N-type resin having a low electric resistance, such s solvent-soluble polyamide, polyvinyl alcohol, polyvinyl butyral, and casein. Only for suppressing the deterioration of charge-holding properties of the photoconductor, the intermediate layer can be formed as thin as possible by using one of the N-type resins for the intermediate layer. For example, the effective intermediate layer can be formed so as to have a thickness of 0.1 .mu.m or under, but thick enough to make a uniform surface of the charge generation layer for covering the rough or contaminated surface of the electrically conductive substrate without causing non uniform distribution of the coating solution of the charge generation layer. Consequently, the intermediate layer should be formed so as to have a thickness of at least 0.5 .mu.m, or preferably 1 .mu.m or over if required.
However, the thin resin layer formed by using one of the resins described above rises the residual potential and changes the electric properties of the photoconductor under the environmental condition of at a low temperature and a low humidity or at a high temperature and as high humidity.
These troubles are due to the changes of electric resistance of the resin layer because these changes are depended on the moisture content of the resin layer that has an affinity for moisture (i.e., hygroscopic properties) and the absorbed water molecules in the resin dissociate into hydrogen ions and hydroxyl ions. These ions are responsible for the ionic conductance which occupies the greater part of the electroconductance.
It has been proposed that a polyamide resin is one of the suitable raw materials for preparing the intermediate layer. In spite of its thickness, for that reason, it has a low electric resistance thereof. The resistance is only slightly changed when its environmental condition is changed. In the case of solvent-soluble polyamide resins which can be used as suitable raw materials, for example, their structures are specified in the documents of Japanese Patent Application Laying-Open No. 2-193152; Japanese Patent Application Laying-Open No. 3-288157; Japanese Patent Application Laying-Open No. 4-31870; and others. In addition, several other documents such as Japanese Patent Application Publication No. 2-59458, Japanese Patent Laying-Open No. 3-150572, and Japanese Patent Application Laying-Open No. 2-53070 disclose that the changes of electric resistance due to the environmental changes are limited by adding an appropriate additive to the polyamide resin. Furthermore, Japanese Patent Application Laying-Open No. 3-145652, Japanese Patent Application Laying-Open No. 3-81778, Japanese Patent Application Laying-Open No. 2-281262, and other documents disclose mixtures of polyamide resin and another kind of resin for adjusting the electric resistance to protect the layer form the effects of the changes of the environmental conditions. However, the intermediate layer comprises the polyamide resin as one of the main components so that the electric properties of the layer can be influenced by a degree of temperature and humidity in the surroundings.
In addition to the polyamide resin, several materials have been proposed as a raw material of the intermediate layer, such as cellulose derivatives (Japanese Patent Application Laying-Open No. 2-238459); polyether urethane (Japanese Patent Application Laying-Open No. 2-115858, Japanese Patent Application Laying-Open No. 2-280170); polyvinyl pyrolidone (Japanese Patent Application Laying-Open No. 2-105349) and polyglycol ether (Japanese Patent Application Laying-Open No. 2-79859).
For keeping a moisture content in the resin layer at a constant against the surroundings, a cross-linking resin has been also proposed as a raw material of the intermediate layer, such as melamine resin (Japanese Patent Application Laying-Open No. 4-22966, Japanese Patent Application Publication No. 4-31576, and Japanese Patent Application Publication No. 4-31577) and phenol resin (Japanese Patent Application Laying-Open 3-48256). The intermediate layer formed by using one of the materials described above is useful when it is formed as an extremely thin film. However, the resistance of the photoconductor, which is the cause of increasing the residual potential, can be increased when it is formed as comparatively a thick film with a thickness of in the order of several .mu.m.
One of the ways for eliminating the problems described above to form the intermediate layer is to use a material having an electric conductivity in the type of electronic conduction instead of ionic one. For this purpose, Japanese Patent Application Publications No. 1-51185, 2-48175, 2-60177 and 2-62861 propose the processes in which the intermediate layers are formed by dispersing the electrically conductive powders such as tin oxide and indium oxide in the resin. In spite of these proposals, however, it is difficult to make a uniform dispersion of the electrically conductive particles in the resin solution to be applied on the conductive layer. Furthermore, the resin solution comprising the particles cannot store well because they are easily segregated and settle to the bottom in the solution, so that the particles tend to protrude as minute projections from a surface of the applied solution during the step of forming the intermediate layer on the conductive substrate. In this case, the intermediate layer with a rough surface can be obtained and the image qualities of the photoconductor can be deteriorated. For improving the image qualities, organic metal compounds are used instead of the above electrically conductive particles. For example, Japanese Patent Application Publication No. 3-4904 and Japanese Patent Application Laying-Open No. 2-59767 disclose the steps of forming the intermediate layer by applying the solution prepared by dissolving the organic metallic compounds and the resin in the organic solvent, nevertheless the solution is not stable enough to provide a uniform surface of the layer. Consequently there are many problems to be solved for producing the photoconductors on a large scale.